Technical Abstract:
The objective of this study was to determine if fecal analysis by NIRS could be used to calibrate for intake determined by the alkane ratio technique in grazing cows. Dry matter intake (DMI; g/kg BW) and diet digestibility (D) was estimated in cows of various breeds in 7 trials over 7 years during different seasons. Nine to 12 cows were used in each trial and on d-1, each received a bolus that delivered a prescribed amount daily of both C32 and C36 n-alkanes as external markers. The bolus was designed to deliver 300 mg of marker each day over about 21 d. In 4 of the trials, actual payout was determined by collecting feces until dosed marker was not detected. Fecal samples were collected
daily from the ground as marked cows defecated in presence of an
observer, lyophilized, scanned by NIRS (400–2500 nm), and analyzed
for n-alkane concentration following ethanolic KOH extraction and
cleanup. Diet D was estimated by the ratio of naturally occurring plant cuticular wax n-alkanes (odd-chain C27-C37) in the forage and feces. Intake was estimated by the ratio of daily dose of external marker and fecal marker concentration corrected for forage C32 or C36. Bahiagrass contained sufficient C31, C33, and C35 for use as markers and concentration in feces varied both within trial and across trials providing variation in D. Calibration equations were developed by PLS regression of DMI
and D on fecal spectra. The equations were then used to predict values on the same spectra to determine if structural bias occurred within the data set among trials and breeds within trial. Differences between alkane estimated DMI (31.6 ± 13.6) and D (65.2 ± 7.0) and NIRS estimated DMI (31.1 ± 8.3) and D (65.5 ± 4.7) were significant (P < 0.001) among trials but not different due to breed within trial. When alkane estimated D was regressed on NIRS predicted D, slopes differed among trials (P < 0.0001) and breed within trial (P < 0.01). However, slopes for DMI differed among trials (P < 0.001) but not breed (P = 0.60). We suggest that NIRS can be used as a double sampling technique with n-alkanes for estimating DMI and diet D on large numbers of animals.